Touch-sensing systems (“touch systems”) are in widespread use in a variety of applications. Typically, the touch systems are actuated by a touching object such as a finger or stylus, either in direct contact, or through proximity (i.e. without contact), with a touch surface. Touch systems are for example used as touch pads of laptop computers, in control panels, and as overlays to displays on e.g. hand held devices, such as mobile telephones. A touch panel that is overlaid on or integrated in a display is also denoted a “touch screen”. Many other applications are known in the art.
There are numerous known techniques for providing touch sensitivity, e.g. by incorporating resistive wire grids, capacitive sensors, strain gauges, etc into a touch panel. There are also various types of optical touch systems, which e.g. detect shadows cast by touching objects onto a touch surface, or detect light scattered off the point(s) of touching objects on a touch panel.
One specific type of optical touch system uses projection measurements of light that propagates on a plurality of propagation paths inside a light transmissive panel. The projection measurements thus quantify a property, e.g. power, of the light on the individual propagation paths, when the light has passed the panel. For touch detection, the projection measurements may be processed by simple triangulation, or by more advanced image reconstruction techniques that generate a two-dimensional distribution of disturbances on the touch surface, i.e. an “image” of everything on the touch surface that affects the measured property. The light propagates by total internal reflection (TIR) inside the panel such that a touching object causes the propagating light on one or more propagation paths to be attenuated by so-called frustrated total internal reflection (FTIR). Hence, this type of system is an FTIR-based projection-type touch system. Examples of such touch systems are found in U.S. Pat. No. 3,673,327, U.S. Pat. No. 4,254,333, U.S. Pat. No. 6,972,753, US2004/0252091, US2006/0114237, US2007/0075648, WO2009/048365, US2009/0153519, WO2010/006882, WO2010/064983, and WO2010/134865.
In touch systems in general, there is a desire to not only determine the location of the touching objects, but also to estimate the amount of force by which the touching object is applied to the touch surface. This estimated quantity is often referred to as “pressure”, although it typically is a force. The availability of force/pressure information opens up possibilities of creating more advanced user interactions with the touch screen, e.g. by enabling new gestures for touch-based control of software applications or by enabling new types of games to be played on gaming devices with touch screens.
Capacitive touch systems may be designed to estimate the application force/pressure of touches, e.g. as disclosed in EP2088501 and U.S. Pat. No. 4,736,191.
The prior art also comprises optical touch systems that use direct imaging by a camera located behind a transmissive panel to detect light that is scattered off objects that touch the panel. The camera thus captures a direct image of the light scattering objects on the touch surface. WO2011/082477 and US2009/0143141 disclose such optical touch systems and propose to measure the size of each touch in the image and compare the current size to a previously detected size of the same touch for establishing a level of pressure of the touch. Thus, as the size increases, force is considered to increase, and vice versa.
Estimation of application force from touch size has also been suggested for use in FTIR-based projection-type touch systems. WO2011/049511, WO2011/049513 and WO2012/002894 all mention that if the area of a detected touch is changed as a function of applied pressure, the application force between the object and the touch surface may be monitored for each touch by detecting changes in touch area over time.
However, the use of changes in touch area for force estimation is limited to certain types of touching objects, since the objects need to be sufficiently soft and flexible to exhibit a variation in size with application force. Many objects, such as fingertips, only exhibit a relatively small deformation with increased application force, and the change in size may be difficult to detect with sufficient accuracy. Furthermore, a change in size of a peak may have other causes than a change in application force, e.g. a change in position of the touching object.